1. Field of the Invention
The present invention relates to a radiation imaging apparatus, its control method, and a radiation imaging system.
2. Description of the Related Art
In recent years, a flat panel detector having an area sensor array in which an amorphous silicon film formed on an insulating substrate such as glass or the like is used as a material and pixels each of which is constructed by a photoelectric converting element and a thin film transistor (TFT) are two-dimensionally arranged has been applied to a medical X-ray radiographing apparatus or the like.
The X-ray radiographing apparatus using such a flat panel detector and a driving method of such an apparatus have been disclosed in, for example, U.S. Patent Application Publication No. 2005/0220270 or the like. An X-ray radiographing apparatus in which signals are read out by matrix-driving flat panel detectors using amorphous silicon has been disclosed in U.S. Patent Application Publication No. 2005/0220270. In the imaging apparatus using flat panel detectors using amorphous silicon, by applying a driving signal to a gate of a thin film transistor, the thin film transistor is driven and the signal is read out. At this time, in addition to an image lag (afterimage) and untransferred charges of the thin film transistor, an offset that is caused by the driving operation of the thin film transistor is added to the read-out signal. A signal to noise ratio of the signal thus obtained deteriorates, causing a deterioration in picture quality. To prevent the deterioration in picture quality, according to U.S. Patent Application Publication No. 2005/0220270, after radiographing image data of the number corresponding to one image was obtained, offset image data of the number corresponding to one image is obtained, and the offset image data of one image is subtracted from the radiographing image data of one image, thereby making offset correction.